LEARN ABOUT…
The ozone layer and ozone depletion
Created: November 2009
Updated: January 2016
The ozone layer
Ozone is created by the action of sunlight on oxygen high in the stratosphere where the air pressure is
very low and sunlight very strong.
Lower down in the stratosphere, ozone is naturally destroyed in reactions with other atmospheric gases
and the ozone layer is a consequence of the creation and destruction processes.
About 300,000,000 tonnes per day of ozone are involved in this cycle[1].
The ozone layer exerts two important influences on the atmosphere:
• the processes that create and destroy ozone generate heat, so that the ozone layer controls the
temperature of the stratosphere; consequently the stratosphere is warmer than the atmospheric
layer immediately below it (the upper troposphere), and
• ozone absorbs the higher energy parts of the ultra-violet (UV) irradiation coming from the Sun
(all of the UV-C and most of the UV-B).
Formation of ozone is controlled by sunlight; this means that most ozone is formed over the Equator
and almost none over the Poles. The ozone in the stratosphere at the Poles was transported there on
stratospheric winds.
Ozone depletion occurs if the processes that destroy ozone are augmented, so that the balance
between its formation and removal is disturbed. This makes the Polar stratosphere particularly
vulnerable (because ozone is not formed there).
A large number of chemical species are involved in the natural ozone removal processes: water,
hydrogen peroxide, hypochlorous acid, nitric acid, methane, carbon monoxide, nitric oxide, nitrogen
dioxide, nitrous oxide, chlorine, chlorine monoxide, hydrochloric acid and bromine monoxide all play a
part. The removal process is also augmented by reactions that occur on the surface of particles; in turn
these are influenced by stratospheric temperatures [2]
The most important parameters now are the chlorine and bromine concentrations and the existence of
stratospheric particles (such as ice clouds).
Learn about the ozone layer and ozone depletion:
The Ozone Layer
Because it is a consequence of an upset to the balance between formation and removal, ozone
depletion is temporary. In the short term, the lower ozone levels that result are regional and seasonal
(like the Antarctic ozone hole) and, in the long term, ozone depletion should cease when the balance is
restored.
Because ozone absorbs solar UV radiation, low ozone levels in the layer coincide with an increase in the
flux of UV-B into the lower atmosphere, although this does not necessarily mean a higher flux at ground
level [3]. The concern is that, if ozone depletion were to persist, the average UV-B flux could increase.
Sources:
1. Chapman S., A theory of upper atmospheric ozone, Mem. Roy. Met. Soc., 3, 103-125, 1930.
2. Stratospheric Ozone, First Report of UK Stratospheric Ozone Review Group, HMSO, London, 1987.
3. The Potential Effects of Ozone Depletion in the United Kingdom, First Report of the UK UVB
Measurements and Impacts Review Group, HMSO, London, 1996.
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